In oil and gas field operations, such as drilling, completion, workover and stimulation (acidizing and fracturing), fluid-loss-control materials (FLCM) are widely used to mitigate formation damage due to the invasion of operation fluids. FLCM can be used to plug pore throat and fractures of the formation and further reduce the invasion of operation fluid into the formation. Regarding to the well penetrated multi layers or acidizing and fracturing of horizontal well, the whole well should be homogeneous acidized or staged fracturing stimulation. One of the most commonly and economically used technologies are temporary plugging acidizing and temporary plugging fracturing. Temporary plugging materials are used to seal the acidized or fractured layers which can prevent the subsequent acid or fracture fluid entering to the acidized or fractured layers, and help to continue the acidizing or fracturing of new layers or sections. These temporary plugging materials can also prevent the formation damage due to the invasion of the formation fluid from reservoir to wellbore.
In order to mitigate the formation damage due to the FLCM or plugging materials, temporary plugging materials (TPM) are usually preferred. TPM will dissolve, degrade and finally disappear after the well operations, which is the goal of petroleum engineers.
There are three types of TPMs including water soluble TPM, acid soluble TPM and oil soluble TPM. Two types of water soluble TPM are usually used, which are water soluble polymer (polyacrylamide, polyvinyl alcohol) and water soluble inorganic salts (potassium chloride, sodium chloride, etc.) mainly used in saturated salt water systems. Acid soluble TMP mainly contains calcium carbonate particles which can be removed by hydrochloric acid after the operations. Oil soluble TPM is resin material, such as C6-C10 resin, asphalt, etc.
The TPMs discussed in the previous section have some drawbacks and limitations. Water soluble TPM requires organic hydrocarbon rather than water as carrying liquid, which would increase the cost of operations and the risk of fire control. It is also required that there is enough water in the formations in the subsequent process which can solve the water soluble TPM, release the temporary plugging section and flow channel of oil and gas. If the formation water is not enough or the formation pressure is so low that cannot drive the water to the temporary plugging sections, additional water is need to be injected into the formation to solve the temporary plugging material. It is common that the injected water cannot solve the temporary plugging material completely and cause additional formation damage. It is required to inject acid to solve the acid soluble TPM and release the plugging after operations. It is not suitable for the acid sensitive formations, while for the non-acid sensitive formations another operation is required and the cost is increased. Some of the acid soluble TPM cannot completely solve even acid is injected. The well tubing will be eroded while injecting the acid into the formation. The oil soluble TPM is not suitable for gas wells and even for oil wells, since the can only contact with the frontier of the temporary plugging section, when formation oil is saturated with the oil soluble TPM, convection and diffusion are dominated resulting in slow dissolve effect which can be considered as another type of formation damage.
Considering the drawbacks of the current TPMs, it is desirable to investigate a new formation temperature dependent TPM that can degrade automatically. The operation fluids are usually injected into the formation with surface temperature which is lower than the formation temperature, and the injected formation temperature would be lowered. It is required that the new TPM is not degradable at the formation temperature and can temporary plug the formation which would prevent the invasion of operation fluids migrating into the formation and mitigate the formation damage while controlling the amount of operation fluid and reducing the cost. The formation thermal energy would transfer to the temporary sections and the temperature of these sections would be increased. This new TPM will be fully degraded and release the gas and oil channel plugged before with zero formation damage.
Different forms of TPMs are required according to the different reservoir characteristic as well as different requirements of field operations. The TPMs are needed to be made into proper shapes, such as granular, powder or fibrous. In the drilling and completion process, it is required to use 1-10 mm diameter particles to seal the leakage pathways in the large cracks; it is required to use 1-100 μm diameter powder and 0.5-1 mm slice to seal the micro fracture; it is required to use different size of granular and fibrous to seal the long well segment with various permeability contrast coefficient in the process of homogeneous acidification. According to multi-section or multi-layer transverse fracturing, it is required to use different size granular and fibrous TPMs. It is required to use suitable ball shape TPM to seam the perforation holes, while larger diameter ball shape TPM is preferred to use in sliding sleeves during the multi hydraulic fracturing.
The well temperatures are different due to the well depth, for example, the most productive zone of DaQing field is from 1000 to 1600 m, and the well bottom temperature is 50-60° C., while in the Tarim field, the well depth of the gas fields such as center Tarim, north Tarim and Kuqa mountain are 5000-8000 m, the bottom temperature is 130-180° C. Considering this situation, it is required that the degradable materials can be degraded at different temperatures.